Radio frequency (RF) switches are key elements of RF systems including communications and radar, where RF switches enable low-loss, low-noise, fast, linear signal routing, impedance tuning, and phase shifting. Linearity is very important over mW to ˜1 Watt system RF power levels.
References 1-13 are listed below and are incorporated herein by reference as though set forth in full. Various RF switches are available, including silicon on Indium (SOI), as described in Reference 1 below, silicon on sapphire (SOS), as described in Reference 2 below, pHEMT FET switches, as described in References 3 and 4 below, and RF MEMS switches, as described in Reference 5 below. Key desired features of RF switches include low insertion loss, high isolation, excellent linearity, power handling, easy integration with conventional semiconductor technologies, high reliability and reasonable size for packaging. The prior art RF switches typically have the following figure of merit (FOM) for Ron·Coff: 250 femtosecond for SOI switches; 448 for SOS switches; 280 for pHEMT switches; and approximately 4 for RF micro-electromechanical system (MEMS) switches.
RF MEMS switches offer the best FOM and have an excellent linearity of greater than 70 dBm. However, while RF MEMS switches have been demonstrated in RF systems with the desired low-loss, low-noise, isolation, linearity, and adequate power handling, issues remain for reaching the desired switching voltage (30-70 V), reliability and packaging. Thus, even after decades of research, RF-MEMS are not ubiquitously found in RF systems. MMIC integration is generally limited because of size and voltage requirements, and applicability to mobile platforms is very difficult because of switching voltage requirements.
RF switches with phase change material (PCM) offer a non-volatile switch option, which eliminates the need for standby power to maintain a SET or RESET status; however, during switching, RF switches with PCM require current pulses of 50-1000 nanoseconds depending on the switch cell size.
The principles of operation of PCM has been known since the 1960s and PCM has been used in rewritable optical DVDs using Ge2Sb2Te5 or (Ag,In)Sb2Te. Lately, PCMs have been developed for non-volatile memory as a future replacement for flash memory in the IC industry by companies such as Micron, Samsung, IBM, STMicroelectronics, and Intel. Two of the latest reports on digital PCMs include References 6 and 7 below.
References 8-13 below describe using PCM in RF applications as RF switches. While Reference 8 mentions using PCM material for RF switches, no RF switch designs are shown. The switch of Reference 9 has a three-terminal layout, consisting of an array of sub-vias, and the switching is performed using external probes. Reference 10 shows via structures with GeTe material, where Ron is 1.1 ohm with Ron/Roff of 3×104, and the switching is performed using external probes. Reference 11 describes an RF switches with PCM.
Reference 12 describes an electric heater 12 integrated on a Si or SiC substrate 13, with the heater 12 and the PCM 16 separated by a SiNx dielectric layer 14, as shown in FIG. 1A. Reference 13 describes a RF-PCM switch layout that has a top heater 20 with a SiO2 passivation layer 22 and a Cr diffusion layer 24 between the top heater 20 and phase change material 16, and a bottom heater 26 with a SiO2 passivation layer 22 and a Cr diffusion layer 30 between the bottom heater 26 and the phase change material 16, as shown in FIG. 1B. Using a SiNx or SiO2 dielectric layer separates the heater from the PCM, but there is significant parasitic capacitive coupling of RF to the heater, which increases the off-state capacitance and reduces the RF isolation.